Add threaded backend sink to DM. Fix a scheduler bug.

The bug is spotted by the new sink. I'd expect more bugs to be revealed
and fixed using the new sink.

Bug: skia:
Change-Id: I6b0c9267079fbd6149004f0ecd55ddb179702588
Reviewed-on: https://skia-review.googlesource.com/109720
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
diff --git a/tools/flags/SkCommonFlags.h b/tools/flags/SkCommonFlags.h
index 96bd6f3..1f0c8d1 100644
--- a/tools/flags/SkCommonFlags.h
+++ b/tools/flags/SkCommonFlags.h
@@ -41,6 +41,8 @@
 DECLARE_bool(forceDeltaAA);
 DECLARE_string(key);
 DECLARE_string(properties);
+DECLARE_int32(backendTiles);
+DECLARE_int32(backendThreads)
 
 /**
  *  Helper to assist in collecting image paths from |dir| specified through a command line flag.